The present invention relates to medical electrical leads generally and more particular to cardiac pacing and defibrillation leads.
When a cardiac pacemaker is implanted into a pediatric patient, the distal end of the lead carrying the electrode is anchored with respect to the heart and the proximal end of the lead is anchored to or near the pacemaker. As the patient grows, the distance between the pacemaker and the electrode at the distal end of the lead generally increases. It has long been recognized that it would be desirable to provide a mechanism to deal with this phenomenon. One proposed mechanism is to provide a sheath around the pacemaker in which the lead may loosely be looped, so that it may be pulled out of the sheath as the patient grows. This approach is disclosed in U.S. Pat. No. 4,013,081 issued to Kolenik.
An alternative approach is to provide a capsule or envelope in which the lead body may be loosely coiled or folded, allowing the lead body to be pulled from the capsule or envelope as the patient grows. This approach is disclosed in U.S. Pat. No. 4,913,587. A third approach is to provide a lead having a lead body which may be elongated, and a conductor therein which permits for some degree of stretch. One such lead is disclosed in European Patent application EP 0004667, in which the lead body is folded back upon itself to provide a concertina-type fold allowing for elongation of the lead body. In this lead, several such concertina folds are provided spaced along the length of the lead to allow for expansion at multiple individual locations along the lead.
In the context of leads employed in cochlear implants, as described in the article, Evaluation of Expandable Lead Wires for Pediatric Cochlear Implants, by ShiAng Xu et al, published in the American Journal of Otology, Vol. 14, No. 2, March, 1993, pages 151-160, an additional mechanism is proposed in which a generally straight, stranded conductor is coiled over a portion of its length and enveloped in an outer tube as a substitute for an envelope or enclosure.